Interesting chemical and physical features of the products of the reactions between trivalent lanthanoids and a tetradentate Schiff base derived from cyclohexane-1,2-diamine

Abstract

The initial use of a tetradentate Schiff base (LH₂) derived from the 2 : 1 condensation between 2-hydroxyacetophenone and cyclohexane-1,2-diamine in 4f-metal chemistry is described. The 1 : 2 reaction of Ln(NO₃)₃·xH₂O (Ln = lanthanoid or yttrium) and LH₂ in MeOH/CH₂Cl₂ has provided access to isostructural complexes [Ln(NO₃)₃(L′H₂)(MeOH)] in moderate to good yields. Surprisingly, the products contain the corresponding Schiff base ligand L′H₂ possessing six aliphatic –CH₂– groups instead of the –CH–(CH₂)₄–CH– unit of the cyclohexane ring, i.e. an unusual ring-opening of the latter has occurred. A mechanism for this Ln^III-assisted/promoted LH₂ → L′H₂ transformation has been proposed assuming transient Ln^II species and a second LH₂ molecule as the H₂ source for the reduction of the cyclohexane moiety. DFT calculations provide strong evidence for the great thermodynamic stability of the products in comparison with analogous complexes containing the original intact ligand. The structures of the Pr^III, Sm^III, Gd^III, Tb^III, and Ho^III complexes have been determined by single-crystal X-ray crystallography. The 9-coordinate Ln^III centre in the molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the bidentate chelating organic ligand, and one oxygen atom from the coordinated MeOH group. In the overall neutral bis(zwitterionic) L′H₂ ligand, the acidic H atoms are clearly located on the imino nitrogen atoms and this results in the formation of an unusual 16-membered chelating ring. The coordination polyhedra defined by the nine donor atoms around the 4f-metal-ion centres can be best described as distorted, spherical capped square antiprisms. The Eu^III, Tb^III, and Dy^III complexes exhibit Ln^III-based luminescence in the visible region, with the coordinated L′H₂ molecule acting as the antenna. Ac magnetometry experiments show that the Dy^III member of the family behaves as an SIM at zero field and under external dc fields of 0.1 and 0.2 T without the enhancement of the peaks’ maxima, suggesting that QTM is not the relaxation path. The Gd^III complex behaves, rather unexpectedly, as a SIM with two different magnetic relaxation paths occurring at very close temperatures; this behaviour is tentatively attributed to a very small axial zero-field splitting (D ∼ 0.1 cm⁻¹), which cannot be detected by magnetization or susceptibility experiments. The prospects of the present, first results in the lanthanoid(III)-LH₂ chemistry are discussed.